1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly to an objective lens having selective transmittance.
2. Description of the Related Art
By virtue of steady efforts to increase the recording capacity of optical discs, a digital video disc (DVD) has been developed which has a recording capacity greater than those of compact discs (CDs). DVD not only has a high recording density (that is, a high track density), but also has a short distance between its disc surface and its information recording surface. Due to such features of DVD, optical pickup devices for DVDs have a difficulty in recording/reproducing information. This is because there may be a spherical aberration caused by a variation in the distance between the disc surface and the information recording surface, a coma aberration caused by a tilting of the optical disc, and an astigmatism caused by defocus.
Spherical aberration serves to cause the intensity of the main lobe of light associated with an information recording medium region to be higher than the intensity of the side lobe of light associated with a region other than the information recording medium region. For this reason, such spherical aberration causes an interference between tracks. Coma aberration and astigmatism serve to make the optical system unstable, thereby degrading optical characteristics. Such spherical aberration, coma aberration, and astigmatism are caused by variations in the distance between the optical disc surface and the information recording surface, the index of refraction, the numeric aperture (NA) of the objective lens, the defocus amount, and the tilt angle of the optical disc.
However, where an optical pickup device uses objective lenses having different numeric apertures to adjust the diameter of a light beam emitted from a light source, it can have access to both CD and DVD.
Although access to both CD and DVD is possible using two objective lenses having different numeric apertures, an improved means has been proposed. For example, an optical pickup device has been proposed which includes an objective lens coated with an SWPi filter to have access to both CD and DVD.
FIG. 1 illustrates the configuration of an optical pickup device for both CD and DVD.
Referring to FIG. 1, the optical pickup device includes a first holographic pickup module (HPM) 101 for outputting a laser beam for DVD, a second HPM 102 for outputting a laser beam for CD, and a beam splitter 103 for transmitting the laser beam outputted from the first HPM 101 therethrough while reflecting the laser beam outputted from the second HPM 102. The optical pickup device also includes a collimator lens 104 for converting the laser beam emerging from the beam splitter 103 into a collimated beam, a mirror 105 for reflecting the collimated beam passing through the collimator lens 104, thereby changing the optical path of the collimated beam, an objective lens 106a for focusing the beam reflected by the mirror 105 onto a desired spot on a disc 107, and an SWP filter 106b for adjusting the numeric aperture of the objective lens 106a. 
The light beam outputted from the first HPM 101 is transmitted through the beam splitter 103, and then converted into a collimated beam by the collimator lens 104. On the other hand, the light beam outputted from the second HPM 102 is reflected by the beam splitter 103, and then converted into a collimated beam by the collimator lens 104. The optical path of the collimator beam from the collimator lens 104 is changed from horizontal to vertical by the mirror 105. In this case, the mirror 105 is a half mirror. The light beam reflected by the mirror 105 is focused onto a desired spot on the disc 107 via the objective lens 106a. The light beam incident to the 106a via the mirror 105 passes through the SWP filter 106b which, in turn, selectively filters components of the light beam corresponding to wavelengths of 650 nm and 780 nm, so as to allow access to both CD and DVD, respectively.
The conventional optical pickup device for both CD and DVD uses an SWP filter coated on the objective lens.
In this case, however, it is difficult to uniformly coat the SWP filter on the surface of the lens having a convex or concave shape. In particular, there is a difficulty in performing a coating process for edge portions of the lens. Furthermore, it is difficult to form a mask to be used in the coating process for the convex or concave lens surface. For this reason, it is difficult to obtain superior optical characteristics.
Thus, there is a difficulty in achieving a desired phase correction, and a degradation in optical characteristics caused by non-uniform coating of the filter.
Therefore, the present invention has been made in view of the above mentioned problems, and an object of the invention is to provide an objective lens of an optical system for an optical pickup device which is divided into two lens portions and provided with an SWP filter inserted between those lens portions while being integral with the lens portions.
Another object of the invention is to provide an optical pickup device to which the SWP filter-inserted objective lens is applied.
In accordance with one embodiment, the present invention provides an optical pickup device for both CD and DVD comprising: a first holographic pickup module (HPM) for outputting a laser beam for DVD; a second HPM for outputting a laser beam for CD; a beam splitter for transmitting the laser beam outputted from the first HPM therethrough while reflecting the laser beam outputted from the second HPM; a collimator lens for converting the laser beam emerging from the beam splitter into a collimated beam; a mirror for reflecting the collimated beam passing through the collimator lens, thereby changing an optical path of the collimated beam; and an objective lens for focusing the beam reflected by the mirror onto a specific spot on a disc, the objective lens having a short-wave pass (SWP) filter inserted in the objective lens and adapted to adjust a numeric aperture of the objective lens.
The objective lens may be divided into two lens portions in a transverse direction to a beam transmitting path, and the SWP filter may be interposed between facing divisional surfaces of the lens portions. The divisional surface of each of the lens portions must be planarized.
The SWP filter may be coated on a selected one of the divisional surfaces or interposed between the divisional surfaces.
In accordance with another embodiment, the present invention provides an objective lens divided into two lens portions in a transverse direction to a beam transmitting path and provided with an SWP filter, the lens portions having planarized divisional surfaces facing each other, respectively.
In this embodiment, the SWP filter may be coated on a selected one of the divisional surfaces or interposed between the divisional surfaces.
In accordance with another embodiment, the present invention provides a method for fabricating an objective lens comprising the steps of: dividing an objective lens into two lens portions in a transverse direction to a path for transmitting a light beam; planarizing respective divisional surfaces of the lens portions; coating an SWP filter on a selected one of the divisional surfaces; and pressing the lens portions against each other using an ultraviolet bonding process so that the lens portions are bonded together.
In accordance with another embodiment, the present invention provides a method for fabricating an objective lens comprising the steps of: dividing an objective lens into two lens portions in a transverse direction to a path for transmitting a light beam; interposing a short-wave pass filter between divisional surfaces of the lens portions; and pressing the lens portions against each other using an ultraviolet bonding process so that the lens portions are bonded together.